(1) Field of the Invention
This invention relates to an improved method for chromizing surfaces of ferritic boiler components and, more particularly, the interior surfaces of iron or steel boiler tubes, pipes and like components to prevent high temperature exfoliation.
(2) Description of the Related Art
The use of chromium in the production of iron and steel is well known in the art. An excellent text on the production and uses of chromium is presented in CHROMIUM, by A. H. Sully, Academic Press Inc., New York, 1954. Chapter 6-"Chromizing", pages 190-222, is particularly directed to the chromizing process.
Chromizing is a thermally activated diffusion process used to produce a high chromium content surface layer on an iron or steel surface. This process is typically used on boiler tubes, pipes, and other boiler components to provide an internal surface which is resistant to exfoliation, i.e., high temperature oxidation of the internal surface with subsequent breaking away or loss of the oxide layer. Boiler components are presently chromized by a process known as pack cementation, a technique that has been widely used throughout industry for many years. An excellent description of the exfoliation problem as it relates to power boilers, the consequences if left unchecked, and the use of the pack cementation chromizing process as a solution thereto is found in an American Society of Mechanical Engineers (ASME) publication 78-JPGC-Pwr-7, titled "Chromizing and Turbine Solid Particle Erosion", A. J. Blazewicz, et al, presented at the joint ASME/IEEE/ASCE Power Generation Conference in Dallas, Tex., U.S.A., on Sept. 10-14, 1978.
The pack cementation process involves placing a chromium containing pack mixture into close contact with the internal surface of the component to be chromized and subsequently heating the entire assembly to an elevated temperature for a specified period of time. In the pack cementation process, a pack mixture comprising chromium, an inert filler (e.g., alumina) and a halide activator (e.g., ammonium chloride) are blended together. To chromize the internal surface of a ferritic boiler component (e.g., tubing or pipe), the tubing or pipe is filled with the pack mixture. The component is then loaded into a controlled atmosphere retort (i.e., reaction vessel) or made into a self-contained retort by the welding of caps onto the ends of the component. The entire assembly is then heated to an elevated temperature and held for a specified length of time to allow the desired chemical reactions and subsequent thermal diffusion process to occur. A typical pack cementation thermal cycle involves holding the entire assembly from one to ten hours in the temperature range from 1800.degree. to 2200.degree. F. A high chromium content surface layer is formed on the internal surface of the component which was in contact with the pack mixture by diffusion of the chromium into the iron. At the end of the thermal cycle, the entire assembly is cooled to room temperature, and the welded end caps removed if necessary, so that the used pack mixture can be removed from the interior. The component is then subjected to a post process cleaning step. The end result of this process is a relatively thick (equal to or greater than 0.002 inches, i.e., 2 mils) chromium diffusion coating on the internal surface of the tubular boiler component.
This diffusion coating nominally consists of a thin outer zone of chromium carbide, with an underlying zone of columnar ferrite characterized by a decreasing chromium concentration with increasing depth of diffusion. Typical "target" (and normally produced) chromized thickness layers are approximately 2 mils (0.002 inches) thick for Croloy 2-1/4 tubing, and approximately 6 mils (0.006 inches) thick for Croloy 2-1/4 pipe. In the tubing, the 2 mil thick chromium rich zone would contain an outer chromium carbide layer about 1/8 mil (0.000125 inches) thick with the underlying columnar ferrite layer comprising the balance of the layer. A similar but somewhat thicker layer was normally observed when chromizing the pipe material, even when using the same general processing conditions, resulting in the 6 mil thick chromium rich zone having an even thinner (i.e., &lt;1/16 mil or 0.0000625 inches thick) outer carbide layer. This difference in chromizing depths and structure is attributable at least in part to the lower carbon content in the surface of the piping than in the tubing that occurs due to the different processes involved in their manufacture.
Baldi (U.S. Pat. Nos. 4,208,453 and 4,209,391) describes various aspects of the above-described pack cementation process for the diffusion coating of steam boiler tubes. Aluminized or chromized coatings can be obtained by the pack cementation processes described therein.
Ramirez (U.S. Pat. No. 3,475,161) describes a method for the formation of cemented carbide surface coatings on metal products, and involves the preparation of a dip coating bath containing an organic solvent, organic binder, and metal or metal/ceramic powder. The method applies a metal or ceramic coating to the surface of a part, and sinters the coating (at 2200.degree. to 2600.degree. F. thermal cycle) for adherence; thus the applied coating itself becomes the surface desired.
The pack cementation technique, while proven to be an effective method for chromizing the internal surfaces of boiler components, has several inherent disadvantages. For example, the pack mix preparation, loading, and removal steps are tedious and time consuming. The gravity loading techniques which are typically employed for filling elongated tubular components require shop areas with high ceilings, or floor pits, or both, to accommodate components as long as 30 feet in length. Moreover, diffusion thermal cycles are relatively long due to the poor thermal conductivity of the pack mix. Finally, large quantities of pack mix can be required since the internal cavity of the component to be chromized must be filled.
Other developments in chromizing have addressed the chromizing of sheet metal or strip through processes that are, in essence, variants of the pack cementation process described above. Seelig (U.S. Pat. No. 3,257,227) describes a method for producing a diffusion coating on metals which uses a powdered composition or mixture as the source of the treating materials, and which is particularly suited to the treatment of metals in the form of long sheets or rolls. In the method, a dry pack mixture is fed into the gap area between sheets during the step of rolling the sheets into a coil, and the coil is subsequently heated to effect the diffusion process. Forand, Jr., et al (U.S. Pat. No. 3,728,149) discloses a method of forming a corrosion resistant coating on steel strip. The method involves applying a chromium-containing powder, such as ferrochrome, to at least one side of the surface of the steel strip or sheet which has, preferably, been coated previously with a volatile liquid having sufficient tackiness characteristics to act as a temporary bonding agent for the powder. A minor proportion of an alkali metal or alkaline earth metal halide is added to the metal powder. The powder coated strip is subjected to a roll compacting operation, or an equivalent means of densification, to develop a more adherent bond between the powder and the strip and is then heated for a time and at a temperature sufficient to produce an adherent iron-chromium alloy on the surface of the strip.
Hauel, et al (U.S. Pat. No. 3,434,871) discloses a method for the preparation of chromium-containing films suitable as resistor coatings on refractories, as conductive thin films, and as corrosion-resistant, thermally stable and oxidation-resistant films. The films are produced by thermal decomposition of a chromium-halide-amine complex; the chromium halide is coated with an organic amine, and if indicated by viscosity requirements, in the presence of an organic solvent such as toluene, chloroform and the like in which the amine complex is soluble. The deposited layer again become the "coating" for the product of interest.
Baker, et al (U.S. Pat. No. 3,775,151) discloses a method for the preparation of chromized ferrous metal sheet material in a high-speed commercial coating line. A non-compacted adherent coating containing a chromium energizer and a particulate source of metallic chromium are applied to the metal sheet. A uniform film or coating of a volatiliizable liquid having a halogen-containing energizer and/or binder therein is applied on at least one surface of the clean dry sheet material, and the resulting wet sheet material is passed through a powder deposition zone where a particulate coating of powdered metallic chromium-containing material is applied thereon. These thin films become an integral part of the component being coated, and are not removed subsequent to processing as is the pack mix in the pack cementation process. As such, this process is more closely allied with vapor plating techniques.
The above methods adapted to the task of chromizing sheet metal or strip involving rolling and/or pressing operations are not suited to the solution of the problems discussed above with respect to the chromizing of ferritic boiler components such as tubes, piping, headers and the like. Therefore, a need exists for an improved method of chromizing a surface of a ferritic boiler component which will overcome these disadvantages in an economical fashion and yet still produce chromized surfaces of acceptable quality and thickness.